Genetic polymorphism of thiopurine S-methyltransferase: molecular mechanisms and clinical importance.

The activity of thiopurine S-methyltransferase (TPMT) is inherited as an autosomal co-dominant trait. In most large world populations studied to date, approximately 10% of the population have intermediate activity due to heterozygosity at the TPMT locus, and about 0.33% is TPMT deficient. TPMT is now one of the most well characterized genetic polymorphisms of drug metabolism, with the genetic basis having been well defined in most populations, providing molecular strategies for studying this genetic polymorphism in human and experimental models. Three mutant alleles, TPMT()2, TPMT()3A and TPMT(*)3C, account for the great majority of mutant alleles in all human populations studied to date. Significant ethnic differences occur in the frequencies of these mutant alleles. Progress in DNA analysis has made it practical to use genotyping techniques for the molecular diagnosis of TPMT deficiency and heterozygosity, thereby avoiding adverse effects that are more prevalent in TPMT-deficient and heterozygous patients prescribed thiopurine medications.